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subnetting
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IP Address Classes
IP addresses are divided into classes to define the large,
medium, and small networks.
Class A addresses are assigned to larger networks.
Class B addresses are used for medium-sized networks,
Class C for small networks,
Class D for Multicasting
Class E for Experimental purposes
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Identifying Address Classes
(0 , 127 reserved)
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IP address types
• IP address could be one of three categories
 Network address
 Host address
 Broadcast address
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Network / Broadcast Addresses
- Network address :
the first IP address in it which all host part bits = 0
- Broadcast address:
the last IP address in the network which all host part bits = 1
no. of host bits
- other addresses are host addresses = 2
-Here are some examples:
Class
A
B
C
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Network Address
12.0.0.0
172.16.0.0
192.168.1.0
-2
Broadcast Address
12.255.255.255
172.16.255.255
192.168.1.255
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Subnet Mask
- 32 bit mask ( 1’s followed by 0’s )
- Used by routers and hosts to determine the number of
network- significant bits ( identified by 1’s )
and host- significant bits in an IP address (identified by 0’s)
- example :
Class
A
B
C
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Network Address
12.0.0.0
172.16.0.0
192.168.0.0
Default subnet mask
255.0.0.0
or
255.255.0.0
or
255.255.255.0
or
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/16
/24
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Octet Values of a Subnet Mask
• Subnet masks like IP addresses can be represented in the dotted
decimal format like 255.255.255.0.
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Subnetting
- Subnetting a network means to use the subnet mask to divide the
network and break a large network up into smaller, more efficient and
manageable segments, or subnets.
- Subnetting is done by taking part of host bits then add it to
the network part
IP
address
Network part
Host part
Subnet
bits
Network part
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Host part
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Subnetting Example
Divide network 192.168.1.0/24 into 4 subnets
Solution: 4 subnets need 2 bits
192.168.1 . 0
192.168.1 . 0000 0000 to 0011 1111
0 - 63
192.168.1 . 0100 0000 to 0111 1111
64 - 127
192.168.1 . 1000 0000 to 1011 1111
128 - 191
192.168.1 . 1100 0000 to 1111 1111
192 - 255
subnet mask is 255.255.255.192 or /26
The first subnet is
192.168.1.0/26
The second subnet is
192.168.1.64/26
The third subnet is
192.168.1.128/26
The fourth subnet is
192.168.1.192/26
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Subnetting Example
Divide network 192.168.1.0/24 into 4 subnets
Solution :
4 subnets need 2 bits
- subnet mask = 255.255.255.192
- interesting octet is 192
- hop count = 256 – 192 = 64
- The first subnet is
192.168.1.0/26
0 - 63
- The second subnet is
192.168.1.64/26
64 - 127
- The third subnet is
192.168.1.128/26
128 - 191
- The fourth subnet is
192.168.1.192/26
192 - 255
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Subnetting Example
Divide network 172.168.0.0/16 into 8subnets
Solution :
- 8 subnets need 3 bits
- subnet mask = 255.255.224.0
- interesting octet is 224
- hop count = 256 – 224 = 32
- The first subnet is
172.16.0.0/19
172.16.0.1-172.16.31.254
- The second subnet is
172.16.32.0/19
172.16.32.1 -172.16.63.254
- The third subnet is
172.16.64.0/19
-The 8th subnet is
172.16.224.0/19
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172.16.64.1-172.16.127.254
172.16.224.1-172.16.255.254
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Subnetting Example
Divide network 10.0.0.0/10 into 4subnets
Solution :
- 4subnets need 2 bits
- subnet mask = 255.240.0. 0
- interesting octet is 240
- hop count = 256 – 240= 16
- The first subnet is
10.0.0.0/12
10.0.0.1-10.15.255.254
- The second subnet is 10.16.0.0/12
10.16.0.1-10.31.255.254
- The third subnet is
10.32.0.0/12
10.32.0.1-10.47.255.254
- The fourth subnet is
10.48.0.0/12
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10.48.0.1-172.65.255.254
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Step method
this method is used only when the subnet
portion does NOT exceed a single octet
(ex. 172.16.0.0 need 6-bits to address hosts
Subnet portion is 10-bits long spanned
through two bytes
and this rule is NOT followed.
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How Many Hosts in the Network
 Design an address scheme for an internetwork and
assign ranges for hosts, network devices and the router
interface
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How Many Networks
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Calculating Addresses: Case 1
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Calculating Addresses: Case 2
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Calculating Addresses: Case 2
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Calculating Addresses: Case 2
 Calculate the address ranges
for sub networks
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VLSM exercise
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